1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device for storing multivalued data or analog data, using a metal-ferroelectrics-semiconductor (MFS) transistor.
2. Description of the Related Art
Volatile semiconductor memory devices are disclosed in Japanese Laid-Open (Kokai) Patent Publication Nos. 3-212889, 3-272086, and 3-272087. In these semiconductor memory devices, stored data is lost upon the turnoff of a power switch.
There are non-volatile semiconductor memory devices in which stored data is held even upon the turnoff of a power switch. Examples of the non-volatile semiconductor memory devices include a mask read only memory (ROM), a programmable read only memory (PROM), an erasable programmable: read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM), and a flash EEPROM. In the mask ROM, data is written in the course of the production of the mask ROM; therefore, a user cannot rewrite the thus stored data. In the PROM, a user can write data; however, the stored data cannot be rewritten. In the EPROM, EEPROM, and flash EEPROM, data can be rewritten, but the write speed is low.
Volatile semiconductor memory devices, such as a dynamic random access memory (DRAM) and a static RAM (SRAM), can be used as non-volatile semiconductor memory devices which are accessible at a high speed. In this case, a battery or the like should be used as a backup power supply for holding stored data.
In recent years, non-volatile rewritable semiconductor memory devices, which are accessible at a high speed and do not require a power supply for holding stored data, have been paid much attention to. Such semiconductor memory devices utilize residual polarization of a ferroelectric material.
One example of the above-mentioned semiconductor memory device is "A Ferroelectric Nonvolatile Memory" disclosed in "1988 IEEE ISSCC". This ferroelectric nonvolatile memory has a memory cell as shown in FIG. 10. The memory cell has a structure in which a capacitor C for storing data is connected to a word line W and a bit line B through a selective transistor Q. The capacitor C includes a ferroelectric material as an inter-electrode insulator. Thus, this ferroelectric nonvolatile memory has the same structure as that of DRAMs except for the use of a ferroelectric material in the capacitor C. Such a memory cell as shown in FIG. 10 can be used as a conventional DRAM by polarizing the capacitor C to either a positive state or a negative state. On the other hand, such a memory cell can store data in a non-volatile manner utilizing residual polarization, when being applied with an inverted electric field exceeding the anti-electric field known as a coercive force of the ferroelectric material.
A non-volatile semiconductor memory device using a metal-ferroelectrics-semiconductor (MFS) transistor in each memory cell is described in "ferroelectric thin film integration technology", p. 261, published by Science Forum, Feb. 1992. As shown in FIG. 11, the MFS transistor is a metal oxide semiconductor (MOS) transistor which has a gate electrode 33 provided above a channel region 31 with a ferroelectric film 32 sandwiched therebetween. The MFS transistor can store data in a non-volatile manner, using residual polarization of the ferroelectric film 32.
The above-mentioned ferroelectric non-volatile semiconductor memory devices have the following disadvantages: Since only one bit can be stored at a time in each memory cell, multivalued data such as ternary data is converted to binary data so as to be stored and analog data is A/D converted to binary digital data so as to be stored. In the case where data is read, D/A conversion, etc. should be conducted.
For the above-mentioned reasons, conventional non-volatile semiconductor memory devices require a large memory capacity for storing image data and the like, which leads a memory system to be large and expensive.